Heart disease is the largest single killer in America killing 960,000 in total, 160,000 between the ages of 35 and 64. More women die from heart disease than men do. The numbers in the industrial world are stunning—58 million Americans and 220 Million Europeans and Asians suffer from heart disease and 5 million Americans and 18 million Europeans have had a heart attack. The cost of heart disease in America is estimated at $286 billion. As the so-called “Baby-Boomers” reach their 60's, it can be expected that the numbers of older people alone will cause the cost burden to increase significantly. The number of Americans affected by heart disease is increasing and will continue to increase as the Baby-Boomers join the Senior Citizen's ranks.
The basic method of assessing heart function is thermodilution, a procedure that involves insertion of a catheter into the pulmonary artery. This method is demanding in terms of cost, equipment and skilled personnel time. This is an invasive process requiring direct access to the pulmonary arterial circulation through the neck or groin via a catheter inserted into the vascular system (vena cava) and heart directly into the pulmonary artery.
This procedure can be fatal and cause other complications including pneumothorax, pulmonary artery rupture, arrhythmia, and severe infections. The Journal of the American Medical Association (“JAMA”) reported in its September 1996 issue that examined data on 5,735 intensive care patients at five U.S. medical centers revealed that those patients who underwent PAC had a 21% higher risk of death within 30 days compared to those who did not undergo the procedure. This study was underscored by the fact that all patients in both groups of the study were matched for disease severity and prognosis.
Cost is another significant consideration is the use of the catheterization. The mean cost of a hospital stay for critically ill patients having a catheterization is $49,300 (according to the above-referred article in JAMA) as compared to a cost of $35,700 for a critically ill patient who did not undergo the catheterization. The catheterization added $13,600 to the total costs of treating these critically ill patients. The costs to the U.S. healthcare system for each 100,000 catheterizations performed each year is estimated at $200 million for catheters and their insertion, and $1,400 million per year for the complications associated with the procedure.
Impedance cardiography was developed in the 1930's. The NASA space program was first to employ impedance cardiography in the '60's as a non-invasive cardiovascular monitor for astronauts in space. It is used, today, in stand-alone devices in medical surroundings. Impedance cardiography is anon-invasive electronic system for measuring impedance changes across the thorax that would be reflective of cardiac function and blood flow from the left ventricle into the aorta.
Impedance cardiography is based upon the principle of a drop in trans-thoracic electrical resistivity that occurs when red blood cells align themselves in a more parallel fashion during ejection of blood into the thoracic-ascending aorta. This phasic drop in trans-thoracic electrical resistivity with each ejection of blood into the aorta is a very small percentage of total trans-thoracic impedance. The signal has both volumetric and velocity contributions to its magnitude and morphology. When high frequency, low energy electricity is passed through tissue, resistance to electrical flow will be lower in wet tissue than in dry tissue. Tissue full of air will have even higher resistivity. The resistance of an electrical conductor is directly proportional to its length and the inherent resistive properties of the conducting material. Resistance is inversely proportional to the mean cross sectional area of the electrical conductor. In other words, a long conductor has more resistance than a short conductor. Thin conductors resist more than thick conductors do. Dry tissue is more resistive than wet tissue. The thorax is an electrical conductor of variable length, volume, and fluid content.
It is possible to safely inject low energy high frequency electrical current through the thorax by placing electrodes on the forehead and distally on the abdomen. One can then measure resistance changes across the thorax. The proximal and distal dimensions of the thorax are defined by encircling electrodes placed at the base of the neck and the lower thorax at the level of the sternal-xiphoid process. It is possible to then measure the resistivity that occurs across a thorax when high frequency constant electrical current is passed between the forehead and abdominal electrode.
The thorax is an electrical conductor that contains several different resistive elements. The lungs contain air and represent a highly resistive component. Skin, subcutaneous fat, and muscle are various thoracic resistive elements, which follow the general principle that the more water, or electrolyte contained per gram of tissue, the lower its resistivity. A third component within the thorax is the heart and great vessels filled with blood, a fluid that may be considered an electrolyte solution with cells added. Blood and salt water are good electrical conductors and therefore have a low resistance. Therefore, the more water in the thorax, the better the conductivity to electrical flow and the lower its resistance. These variations in conductivity can be sensed and reduced to a set of cardiovascular measurements.
Although the need for clinical care is necessary and important, increasingly it is becoming apparent that it is also important to prevent cardiovascular diseases. There is a concurrent need for better management of cardiovascular health.
Even more generally, a need exists for a low cost, system and method for medical testing that expands widespread use of limited medical expertise.